1. Field of Invention
The present invention relates to a Sagnac interferometric distributed fiber-optic leakage detection device. More particularly, the present invention relates to a linear Sagnac interferometric distributed fiber-optic pipeline leakage detection device.
2. Description of Related Art
In general, oil or natural gas is transferred from place to place via pipelines. When pipelines are used for transferring oil or natural gas, a portion of the pipeline may be buried beneath the ground to prevent damages due to accidents. However, if a particular section of the buried pipeline has a crack and results in some leakage, finding the exact location of leakage is very difficult. Hence, auxiliary equipment must be installed to pinpoint the leaking point.
FIG. 1 is a block diagram showing a conventional optical fiber leak detection system. As shown in FIG. 1, a transmission pipeline 16 is supposed to carry natural gas, for example. A light generator 10 produces a beam of light that enters a 2*2 optical fiber coupler 12 via an inlet at point A. The beam of light passes out from an outlet port at point C and then enters a sensing optical fiber 14. The sensing optical fiber 14 runs along the natural gas pipeline 16 and then loops back to an inlet point D (the sensing optical fiber 14 has a looping architecture) of the 2*2 optical fiber coupler 12. As light passes through the leaky section of the gas pipeline 16 (cracked section), pressure perturbation (an acoustic signal) that initiates at the leakage source results in a phase change of guided light inside the sensing optical fiber 14.
Light returning from sensing optical fiber 14 exits from outlet point B of the 2*2 optical fiber coupler 12 into a photo detection device 18. Photo detection device 18 transforms incoming optical signals into electrical signals and outputs to a spectrum analyzer 20. Spectrum analyzer 20 picks up the electrical signal to assess any frequency shift so that the exact location of leak can be pinpointed.
However, the looping optical fiber has to be installed along a narrow-diameter gas pipeline, and bending in a portion of the optical fiber loop is unavoidable. When light passes along these bends of the sensing optical fiber, a portion of the light beam will be permanently lost (known as bending loss). Furthermore, half of the length of the optical fiber in the sensing loop must be clad by a special material for shielding from unwanted external acoustic fields. Yet, cladding the optical fiber with a special material increases the cost of laying sensing optical fiber along pipeline.
In addition, optical fiber is sensitive to temperature or environmental changes so that the polarization of light passing through an optical fiber may be changed. Hence, a phase modulator (such as the oscillator 22 shown in FIG. 1) serving as a quadrature conditioner is often required to prevent from the polarization induced signal fading problems.
Accordingly, one object of the present invention is to provide an on-line Sagnac interferometric distributed fiber-optic leakage detection device. The device uses a sensing optical fiber having a linear architecture so that the sensing optical fiber has no bends. Furthermore, a 3*3 optical fiber coupler is used as a beam splitter. Consequently, sensitivity in detecting a leakage point somewhere along an oil or gas pipeline is greatly increased.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an on-line Sagnac interferometric distributed fiber-optic leakage detection device. The device includes a light generator, an optical fiber coupler, a hollow pipeline, a Faraday rotator mirror, a sensing optical fiber, a delay optical fiber, a photo detection device and a spectrum analyzer. The light generator produces a beam of light to the optical fiber coupler. The first optical input/output port of the optical fiber coupler is coupled to the light generator. The optical fiber coupler partitions the input intensity of light going into the first optical input/output port of the coupler into three equal portions. The hollow pipeline is used for transporting liquid or gas. The Faraday rotator mirror mounted at the end of the sensing optical fiber which is installed inside the hollow pipe for compensation of the polarization of the reflected light. The sensing optical fiber is coupled to a second optical input/output port of the optical fiber coupler and the Faraday rotator mirror for transmitting light coming from the second optical input/output port of the optical fiber coupler. The sensing optical fiber transmits light coming from the second optical input/output port of the optical fiber coupler. The delay optical fiber is coupled to the third optical input/output port and the fourth optical input/output port of the optical fiber coupler for transmitting light coming from the third input/output port of the optical fiber coupler. The photo detection device is coupled to the fifth optical input/output port of the optical fiber coupler for receiving light variation signals coming from the fifth optical input/output port of the optical fiber coupler. Light variation signals are converted to electrical signals by the photo detection device. The spectrum analyzer is connected to the photo detection device for receiving and analyzing electrical signals. The sensing optical fiber is a linear structure so that there are no bends for contributing to bending losses.
This invention also provides an on-line Sagnac interferometric distributed fiber-optic leakage detection method. First, a light beam is generated. The beam of light is split into a first sensing light beam and a second sensing light beam. The first sensing light beam follows a first route passing close to any leakage point and then returns to a measuring station. The measuring station registers a first optical field signal given by the formula
EI=E0exp{j[Wct+xcex5"PHgr" sinxcfx89a(txe2x88x92xcfx841)+xcfx861]}.
The second sensing light beam follows a second route passing close to the leakage point and then returns to the measuring station. The measuring station registers a second optical field signal given by the formula
EII=E0exp{j[Wct+xcex94"PHgr" sin xcfx89a(txe2x88x92xcfx842)+xcfx862]}.
When the first light beam and the second light beam is mixed inside the measuring station, an interference signal is generated. The interference optical signal is converted into an electrical current signal. The current signal is given by the formula
i=xcex7I0{exp[j{Wct+xcex94"PHgr" sin xcfx89a(txe2x88x92xcfx841)+xcfx861}]+exp{j[Wct+xcex94"PHgr" sin xcfx89a(txe2x88x92xcfx842)+xcfx862]}}2.
The current signal is separated out into a direct current signal and an alternating current signal. The direct current signal portion is filtered away so that the residual current signal is proportional to the alternating current signal portion cos{xcex94"PHgr"[sin xcfx89a(txe2x88x92xcfx841)xe2x88x92sin xcfx89a(txe2x88x92xcfx841)]+(xcfx861xe2x88x92xcfx862)}. Finally, the alternating signal is simplified. When the modulated frequencies satisfy the condition             f      a        =                            ω          a                          2          ⁢          π                    =                        N                      τ            d                          =                  NC                      2            ⁢                          n              ⁢                              (                                  l                  -                  r                                )                                                          ,
and the output signals become null. At this time, the modulated frequency ƒa is called as the null frequency ƒnull. Measuring the frequency shift of the null frequencies between absence of leakage and presence of leakage can find the location of the leakage point.
In brief, this invention provides an on-line Sagnac interferometric distributed fiber-optic leakage detection device and method that utilizes linear sensing optical fibers. By using linear sensing optical fibers, losses due to bending are reduced and the sensing optical fibers are more easily fitted inside a narrow pipeline. Furthermore, special cladding material is no longer needed to shield the sensing optical fibers from unwanted acoustic field, thereby reducing material cost for installing the measuring device.
In addition, a conventional leakage detection system uses a 2*2 optical fiber coupler as a beam splitter and so an additional phase modulator is required. This invention uses a 3*3 optical fiber coupler to serve as a beam splitter and an Faraday rotator mirror so that the signal-fading problems due to induced polarization are resolved. Ultimately, sensitivity in detecting the location of a leakage point along any oil or gas pipeline is greatly increased.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.